Important ultraviolet wavelength regions absorbed by sunscreen cosmetics are the UV-A region (320-400 nm) and UV-B region (290-320 nm). It was believed that the ultraviolet light in the UV-A region (320-400 nm) darkened the skin but it would not cause sunburn and accelerate aging of the skin as the ultraviolet light in the UV-B region (290-320 nm) would. However, in recent years, it has been made clear that, whereas the ultraviolet light in the UV-B region (290-320 nm) only reaches the surface part of the skin, the ultraviolet light in the UV-A region (320-400 nm) reaches the deeper part of the skin and induces not only skin aging but also skin cancer.
Ultraviolet absorbents for cosmetics that have been used up to the present are structurally categorized into (1) benzoic acid derivatives, (2) cinnamic acid derivatives, (3) benzophenone derivatives, (4) dibenzoylmethane derivatives, and (5) salicylic acid derivatives. In recent years, ultraviolet absorbents of (2) and (4) are frequently used.
However, the ultraviolet absorbents listed above each have problems from a practical point of view. For example, 2-ethylhexyl-p-dimethylaminobenzoate, an example of (1) benzoic acid derivatives, is a transparent liquid and has the advantage of being easy to handle; however, it and its derivatives are questionable in terms of safety and therefore are not used in recent years. Also, its peak absorption wavelength is near 290 nm and it absorbs only the ultraviolet light in the UV-B region.
Among (2) cinnamic acid derivatives, 2-ethylhexyl p-methoxycinnamate is the most frequently used ultraviolet absorbent in the sun care cosmetics currently available commercially. Its maximum absorption wavelength is near 310 nm and its absorption region does not reach the UV-A region. Also, sunlight degenerates it and therefore it has problems with staining and also with instability of the ultraviolet protection effect.
As for (3) benzophenone derivatives, 2-hydroxy-4-methoxybenzophenone, for example, absorbs both the UV-A and UV-B regions and exhibits relatively good solubility in external preparation base agents; however, its maximum absorption wavelength is rather close to the UV-B region and the absorbance is not very high. Also, in recent years, its basic structural skeleton (benzophenone) has been implicated as an environmental hormone and its use has been avoided.
Among (4) dibenzoylmethane derivatives, 4-tert-butyl-4′-methoxybenzoylmethane is frequently used for external preparations. Its maximum absorbance is at around 360 nm and the absorbance level is high, and therefore it is a superior ultraviolet absorbent in the UV-A region. However, it has a problem in photostability, and it exhibits poor compatibility with oil components in external preparations and therefore only a small amount can be added.
Among (5) salicylic acid derivatives, octyl salicylate is used. It has a maximum absorption wavelength in the UV-B region, and it is in an oil form and exhibits superior compatibility with paraffin oil and such; however, since its absorbance is low, it is not put to practical use much.
Therefore, 2-ethylhexyl-p-methoxycinnamate from (2) is often used for the UV-B region and 4-tert-butyl-4′-methoxybenzoylmethane from (4) is often used for the UV-A region. In recent years in particular, there is an increasing demand for ultraviolet absorption in the UV-A region.
Some ultraviolet absorbents are sticky, which can be a significant problem when they are blended into sunscreen cosmetics, for which the sensation during use is deemed important. That is, when the blend ratio of the ultraviolet absorbent is increased for the purpose of a higher ultraviolet absorption effect, the sensation during use dramatically worsens. Therefore, there are cases where the desired blend ratio cannot be achieved with the desired ultraviolet absorbent.
Also, generally, a practice of blending multiple ultraviolet absorbents into a sunscreen cosmetic is done for the purpose of securing a broad absorption range.
However, in the case of sunscreen cosmetics containing an ultraviolet absorbent, the sensation during use may decrease. Since a superior sensation during use is an important element strongly required for cosmetics, the practice of blending in multiple ultraviolet absorbents, particularly 3, 4 or more different types, is not usually done, even for the purpose of securing a broad absorption range.
Meanwhile, dibenzoylmethane derivatives, which are blended into cosmetics as ultraviolet absorbents, lose some of their UV absorption capacity when exposed to ultraviolet light. Technology that uses both a benzyolmethane derivative and α-cyano-β,β-diphenylacrylate, which is another ultraviolet absorbent, has been developed in order to control this phenomenon and secure the photostability (Patent Document 1).
Also, in the case of a cosmetic that uses both ultraviolet absorbents, a 1,3,5-triazine derivative and a dibenzoylmethane derivative, the 1,3,5-triazine derivative chemically degrades significantly by ultraviolet irradiation under the presence of 4-tert-butyl-4′-methoxydibenzoylmethane. Therefore, technology that additionally uses α-cyano-β,β-diphenylacrylate to secure photostability of the 1,3,5-triazine derivative and dibenzoylmethane derivative has been developed (Patent Document 2).
However, there is a problem in the technology that secures photostability of the ultraviolet absorbents themselves by using multiple ultraviolet absorbents. Because many of the solid ultraviolet absorbents have low compatibility with, and are hardly soluble in, the oil components that are the base agent of the cosmetics, therefore, if they are to be stably blended into cosmetics, a large amount of specific oil components having superior compatibility needs to be added. The presence of specific oil components in large quantities would be a cause of a reduction in the stability of the cosmetic (particularly the emulsification stability of oil-in-water emulsified cosmetics) and a reduction in the sensation during use.
As described thus far, using multiple ultraviolet absorbents together is a cause for a reduction in the stability and in the sensation during use of the cosmetic, and therefore it is not a usual practice to blend multiple (particularly four types or more) different ultraviolet absorbents into a sunscreen cosmetic.
Also, there are few reports on the emulsification stability (particularly the dissolution stability of the ultraviolet absorbents at lower temperatures) in cases where four or more ultraviolet absorbents are blended into an oil-in-water emulsified composition.